1. Field of the Invention
The present invention relates generally to a system for processing video signals from a television camera, a video camera or the like and more specifically to a system for processing video signals to detect changes in the image data. In particular, the invention relates to a video signal processing system for use in a security monitor system for monitoring a predetermined area to detect abnormalities in the monitoring area.
2. Description of the Background Art
Security monitor systems have been widely employed in various places, such as banks or super markets for preventing theft. In such security monitor systems video cameras, television cameras or still cameras are employed for picking-up the video image in the area to be monitored, which area to be monitored will be hereafter referred to as the "monitoring area". In the following disclosure, the video camera, television camera, etc. for continuously picking-up the image of the monitoring area, will be generally referred to as the "camera".
In a typical security monitor systems, video signals produced by the camera are monitored to detect changes in the video data for detecting a change of condition in the monitoring area. The video signals are reproduced on a monitor screen, such as a cathode ray tube (CRT) and/or recorded by means of a video tape recorder (VTR). Visual monitoring requires an operator to monitor the screen for detecting a change of condition in the monitoring area. When the video signal is recorded on a VTR tape for later viewing in the event of a theft, etc., the recording time is strictly limited. Therefore, in order to record all of the video signal during the period of monitoring requires extensive lengths of VTR tape. In some security monitor systems, the recording time has been expanded by only recording the video data of every other so many fields. However, in this case, when a change of the condition in the monitoring area occurs outside of the recording interval, the change cannot be recognized.
In order to solve the aforementioned problems in the prior art, there has been proposed a video signal processing system which can automatically monitor video signals for detecting changes in the video data of the video signals. Such a system has been disclosed in the Japanese Utility Model First (unexamined) Publication (Jikkai) Showa 53-1120226. In the disclosed system, video data is compared with video data in the immediately preceding field. The number of changes in the corresponding pixels of the successive fields is counted. The counted value is compared with a preset threshold value for detecting a change in the condition in the monitoring area. That is, when the counted value is greater than the threshold value, a determination can be made that the video data is changed and thus a determination can be made that a condition in the monitoring area is changed.
Such a system will operate automatically for detecting a change of the video data. On the other hand, in such system, it is extremely important to prevent the system from erroneously detecting a change of the video data due to signal noise being superimposed on the video signal. For example, when the flicker cycle period of a fluorescent lamp in the monitored area is not synchronous with the image pick-up cycle period, flicker "noise" can be superimposed on the video signal. This flicker noise tends to cause a change in the video data for a relatively large number of pixels. In such a case, an erroneous detection of the change in video data is inevitably made although the condition in the monitoring area is unchanged. To avoid the effects of noise it is preferred to set the threshold value at a sufficiently high value. On the other hand, a greater threshold value to be compared with the counted value will lower the sensitivity to detecting changes in the video data. Therefore, when a relatively small, but actually occurring change in the video signal occurs, such a small change can be overlooked due to dulled sensitivity.